1. Field of the Invention
The present invention relates to an exposure apparatus and more particularly, a scanning exposure apparatus for transferring a pattern on a mask onto a photosensitive substrate during the manufacture of semiconductor devices, liquid crystal devices, or the like.
2. Discussion of the Related Art
Photolithography is one of the important processes in manufacturing semiconductor devices, liquid crystal display devices, or the like. In photolithography, a pattern on a mask is transferred onto a photosensitive substrate by exposure through a projection optical system, for example. In a projection exposure apparatus used in such a photolithography process, recent demand for exposing a larger chip pattern on the mask onto the photosensitive substrate has led to development of a scanning projection exposure apparatus in which the mask and the photosensitive substrate are synchronously moved and scanned by a spatially fixed exposing radiation. In general, the exposing radiation has a slit-like or circular illumination field. The mask and the photosensitive substrate are mounted on a mask and a substrate stage, respectively, and the mask stage and substrate stage are moved in predetermined directions.
The scanning exposure operation generates undesirable reaction forces within the exposure apparatus, and as a result, various components of the exposure apparatus may be inclined or deformed. The reaction forces are normally generated during acceleration and deceleration periods of the mask and substrate movement, but not during constant motion of the mask and substrate stages. To solve this problem, a method of generating counter forces has been proposed. In this method, suitable driving signals are amplified and supplied to actuators, which exert appropriate forces to the apparatus to cancel the reaction forces. FIG. 6 plots the driving signal level versus time during scanning exposure operation. The graph shows that driving signal S1 has positive and negative peaks at an acceleration period Ap and a deceleration period Dp, respectively.
In the conventional method above, however, a driving signal S1 of small magnitude is supplied to the actuators through the amplifiers even during a constant-motion period Cp (FIG. 6). This is because the amplifiers connected to the actuators are always held in the ON-state. Thus, even when the thrust command signal (driving signal) S1 should not be generated (during a constant-speed operation), there is a possibility of generating the thrust command signal S1 due to noise in the amplifiers, or due to fluctuation in velocity during the constant-speed motion of the stages. As a result, unnecessary forces may be generated within the apparatus, in particular, in support members that supports the mask stage and the substrate stage. These forces may degrade scanning accuracy (or alignment accuracy between the mask and the substrate).